Curve with different scale streched
Each curve vertex has a radius. When you scale your curve in Object mode, the radius doesn't change, but if you apply the scale, the radius will change to the curve object scale before applying, and it will affect the objects that are parented to the curve, like your cube.
You can check the vertices radius in the N panel &gt; Item &gt; Radius (or Mean Radius if you've selected several vertices). You can bring back the value to 1. You can also right click (Contexte Menu) &gt; Set Curve Radius, type the value you want and press Enter.